The present invention relates to the jacketing of objects made of steel, particularly pipes, tubing or the like under utilization of synthetic material for the purpose of jacketing.
Numerous methods are known to provide metal objects particularly steel pipes with a protection against corrosion. This is particularly true in those cases in which subterranean installation of such tubing or other metal objects is envisioned and here a thermoplastic cover made of polyethylene, polyvinylchloride or polypropylene has been found highly suitable. The particular jacket is bonded adhesively to the steel pipe which is the usual method for attachment. For covering the tubing with a synthetic material one can however use a seamless hose which for example is extruded by means of an annular nozzle onto and around the tube, i.e. the tube to be jacketed is passed through the annular nozzle whereupon the latter, through the extrusion process, coats the passing tube. Prior thereto the tubular surface may have been coated with an adhesive.
Alternatively it is known to provide a synthetic jacket by means of a foil-like ribbon which is helically wrapped around the tube. It is also known to spray powder onto the tube or pipe which powder is then melted to obtain a cohesive layer. In addition to the thermoplastic coating and layering method as outlined above it is also known to coat tubing with an epoxy resin for purposes of corrosion protection with a total thickness of between 0.5 and 0.8 mm.
In order to improve adhesion of the jacketing and coating layer to the steel pipe surface European Patent No. 85 73 0169 proposed to chromatize the surface of steel pipes following spray cleaning (blasting) by means of steel wire grains or granules. Subsequently the tubing so cleaned is completely dried and heated to a temperature of at least 200 C whereupon epoxy resin is applied to form a layer with a thickness of at least 0.05 mm. Prior to curing the epoxy resin an ethylene copolymer is applied to its surface and should be used as a bonding layer and subsequently a polyethylene is applied as an outer cover being bonded to the epoxy resin by means of the ethylene copolymer. These features improve the tube's resistance against corrosion and oxidation while the strength of the layer as such is increased; this is a particularly important feature in those cases where there is a significant moisture at warm temperature. The adhesion between various layers as mentioned is quite adequate from many points of view, but the transition from the chromate layer to the synthetic layer coating is still to be regarded a weak point.